About the synthesis of a diol by reductive coupling reaction of a carbonyl compound, it is known that a metal compound or metal salt, such as magnesium amalgam, aluminum amalgam, samarium iodide or vanadium chloride, functions as a reducing agent (Non-Patent Document 1).
However, the metal compound or metal salt is expensive and harmful, and further it is necessary to conduct the reaction in a water-free organic solvent in the atmosphere of an inert gas. For this reason, the reaction using the metal compound or metal salt was very unsatisfactory for a simple and environment-friendly reducing process. Known is also a process of conducting the reaction in an organic solvent, using metallic calcium as a reducing agent (Patent Document 1).
Secondary alcohols and diketones containing an aryl group and/or an alkyl group are widely used as intermediate compounds for medical supplies, colorants, and others. It is necessary to synthesize these compounds by an environment-friendly and safe process.
For the synthesis of a secondary alcohol through reductive reaction of a ketone compound, it is known that a metal hydride containing boron or aluminum, such as NaBH4, LiBH4, LiAlH4 or Zn (BH4)2, functions as a reducing agent. However, the metal hydride is expensive and harmful, and further these metal hydride has a drawback that the metal hydride can be used only in a dry atmosphere and a dry solvent containing no water since the metal hydride dislikes the presence of water extremely. Besides, known is a process of causing polymethylhydrosiloxane to react with a ketone in the presence of a catalytic amount of tetrabutylammonium fluoride to reduce the carbonyl group of the ketone, thereby yielding an alcohol compound (Patent Document 2).
In 1970, H. B. Bartl et al. disclosed that the crystal of 12CaO.7Al2O3 (referred to as “C12A7”) has a unique crystal structure that two out of 66 oxygen ions present in a cell containing two molecules thereof undergo clathration, as “free oxygen”, into spaces in cages present in the crystal (Non-Patent Document 2). Thereafter, it was made evident that the free oxygen ions can be substituted with various anions. In particular, when C12A7 is held in an intensely reducing atmosphere, entire free oxygen can be substituted with electrons. C12A7:e−, wherein free oxygen is substituted with electrons, can be regarded as an electride.
Electride compounds are based on an idea suggested unprecedentedly by J. L. Dye (Non-Patent Document 3). No electride compounds were realized until a compound containing a crown ether as a cation and an electron as an anion, and other compounds were produced. It is known that an electride exhibits electroconductivity through the hopping of an electron contained as an anion. Thereafter, some organic electrides were found out. However, all of these compounds are stable only at a temperature as low as about −100° C. or lower, and are remarkably unstable compounds reactive with air or water.
The inventors filed, as a patent application, an invention relating to electroconductive C12A7 and analogue compounds thereof, and a production process thereof (Patent Document 3). The inventors found out that a C12A7 compound having an electroconductivity of 103 S/cm or less is yielded by annealing C12A7 monocrystal at high temperature in a vapor of an alkali metal or alkaline earth metal, ion-implanting an inactive ion such as Ar into C12A7 monocrystal, or solidifying C12A7 monocrystal directly from a melt in a reducing atmosphere. An invention relating thereto was filed as a patent application (Patent Document 4). Furthermore, the inventors succeeded in yielding C12A7 exhibiting metallic electroconductivity by annealing C12A7 monocrystal in a vapor of metallic titanium (Ti), and then filed, as a patent application, the production process of C12A7, and a usage thereof as an electron-releasing material (Patent Document 5).
These C12A7 compounds, which exhibit a good electroconductivity, are compounds wherein all of free oxygen ions are substituted with electrons, are each substantially represented by [Ca24Al28O64]4+ (4e−), and can be regarded as inorganic electride compounds (Non-Patent Document 4).
Electrons undergoing clathration into C12A7 electrides are loosely bonded to cations, so that the electrons can be taken into the outside by applying an electric field thereto or by a chemical means. It appears that the electrons taken into the outside can be used in reductive reaction. However, unknown is an example in which electrons undergoing clathration into a C12A7 electride are applied directly to reductive reaction.
Non-Patent Document 1: G. M. Robertson, Comprehensive Organic Synthesis 3, 563 (1991)
Non-Patent Document 2: H. B. Bartl, T, Scheller and N. Jarhrb, Mineral Monatsh, 1970, 35, 547-552
Non-Patent Document 3: F. J. Tehan, B. L. Barrett, J. L. Dye, J. Am. Chem. Society, 96, 7203-7208 (1974)
Non-Patent Document 4: S. Matsuishi, Y. Toda, M. Miyakawa, K. Hayashi, T. Kamiya, M, Hirano, I. Tanaka and H. Hosono, Science, 301, 626-629 (2003)
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-265391
Patent Document 2: JP-A No. 10-87530
Patent Document 3: WO 2005/000741A1
Patent Document 4: JP-A No. 2004-26608
Patent Document 5: WO 2007/060890A1